Apparatus and method for predetermined component placement to a target platform

ABSTRACT

The present invention relates generally to assembly techniques. According to the present invention, the alignment and probing techniques to improve the accuracy of component placement in assembly are described. More particularly, the invention includes methods and structures to detect and improve the component placement accuracy on a target platform by incorporating alignment marks on component and reference marks on target platform under various probing techniques. A set of sensors grouped in any array to form a multiple-sensor probe can detect the deviation of displaced components in assembly.

CROSS-REFERENCES TO RELATED APPLICATION

This application is a continuation of U.S. application Ser. No.11/351,418 of the same title filed Feb. 10, 2006, which claims priorityto Provisional Application Ser. No. 60/652,217 filed Feb. 11, 2005, bothof which are incorporated by reference.

BACKGROUND OF THE INVENTION

The present invention relates generally to assembly techniques.Alignment and probing techniques to improve the accuracy of componentplacement during assembly are described. More particularly, theinvention includes methods and structures to detect and improve thecomponent placement accuracy on a target platform by incorporatingalignment marks on components and reference marks on target platforms.The proper alignment of the marks is assessed with various probingtechniques. A set of sensors grouped in an array to form amultiple-sensor probe can detect the deviation of displaced componentsin assembly. Merely by way of example, the invention may be applied toplace packaged devices onto electronic substrates for the manufacture ofelectronic systems. However, it should be recognized that the inventionhas a much broader range of applicability.

Electronic devices have proliferated over the years. As the complexityand the operation speed of integrated circuits (IC) increase, it is notunusual to see an increasing number of devices with pin-counts exceedinghundreds or even a thousand. For example, high-speed designs requiresmany power and ground pins. Differential pairs are replacing thesingle-end signals at the input and output pins (I/O) of a device tomeet the signal integrity requirements. In addition, as system-on-a-chipbecomes a reality, more and more pins are added to the device I/O tosupports more functions. Many if not all of these tend to increase thenumber of pins in a packaged device or component.

As the device pin-count increases, the pin pitch of the device tends todecrease to limit the package size. The reduced pin-pitch poses achallenge for the placement equipment to place components accurately ona target platform, such as a printed circuit board (PCB), especially ifthe pin pitch is smaller than 0.5 mm.

Conventional surface-mount equipment use the Cartesian coordinates atthe center of a target land pattern as a reference point to place acomponent on a PCB. There is no feedback to monitor the accuracy of thecomponent placement. Without proper feedback, the accuracy of thecomponent placement is uncertain. Actually, the accuracy of thecomponent placement is influenced by the imperfectness in the package'soutline, the deviation of the component's contact array from an idealgrid location, the imperfectness in PCB mounting references, the agingand the intrinsic tolerance of the placement equipment, and so on. Asthe accumulative error is getting closer to the pitch size of thecontact array, placing a component accurately on a PCB is becoming agreater challenge.

It is not uncommon to encounter component placement problems in asurface mount assembly line, especially for the placement of fine-pitchcomponents. For example, if a BGA component is inaccurately placed on aPCB, it could cause the BGA's contact array to deviate from the idealland pattern location, resulting in either inadequate soldering orsolder bridging to adjacent pads on the PCB. A rework to fix theseproblems is tedious and expensive. It is even worse for the rework of apricy, high pin-count component on a high density PCB.

Furthermore, manufacturers frequently use sockets to house high-end,high pin-count chips on motherboards. This enables users to chooseproper speed grade components or to perform speed upgrade at field.However, there is no handy method for users or manufacturers to monitorif a chip has been properly inserted in the socket or if the chip is ingood contact with the receptacle inside the socket.

Thus, it is seen that techniques for detecting and improving componentplacement accuracy and for detecting the contact status are desirable.

BRIEF SUMMARY OF THE INVENTION

The present invention relates in general to assembly techniques.According to the present invention, the alignment and probing techniquesto improve the accuracy of component placement in assembly aredescribed. More particularly, the invention includes methods andstructures to detect and improve the component placement accuracy on atarget platform by incorporating alignment marks on components andreference marks on target platforms. Proper alignment is assessed withvarious probing techniques. A set of sensors grouped in an array to forma multiple-sensor probe is used to detect the deviation of displacedcomponents on a target platform. Merely by way of example, the inventionmay be applied to place components onto electronic substrates for themanufacture of electronic systems. However, it should be recognized thatthe invention has a much broader range of applicability, such theprecision alignment of a set of different objects.

In a specific embodiment, the present invention provides a solution forICs or packaged devices which have a set of one or more alignment marksin predetermined spatial regions on the IC or packaged device. Dependingon the embodiment, the packaged device, containing a plurality of I/Ocontacts, can be an integrated circuit device encapsulated in a suitablematerial, such as plastic (e.g., epoxy), ceramic (e.g., aluminumdioxide) or other material, and a plurality of bonding pads connected toits I/O contacts. The packaged device can be a multiple-IC stackeddevice, a multiple-package stacked device, or a multiple-chip carrier.The packaged device can also be an integrated circuit device laminatedwith an anisotropic conductive elastomer (ACE) membrane as aninterconnect interface for external connections, and so on. One or moresensors monitor the placement of the set of alignment marks of such apackaged device on a target substrate or platform to determine if theexternal contacts on the device are accurately placed relative to a landpattern on the target substrate or platform, such as a printed circuitboard, mother board, ceramic board, bare die IC, or other packageddevice or component. If the packaged device is inaccurately placed, thealignment mark can also be used to obtain feedback to adjust the deviceposition deviation. For simplicity, the ICs and packaged devices will bereferred to as components.

In a specific embodiment, an alignment mark is a reference area on acomponent that an external probe can use to monitor the accuracy of acomponent placement on a target platform. The alignment markincorporated on the component can be a conduction path connecting a topsurface area to a bottom surface area of the component as a directalignment mark, or it can be a different conduction path connecting twosurface areas on the bottom of the component as an indirect alignmentmark. Besides being a conduction path within the component, thealignment mark can also be a simple surface marking on the component,depending upon the probing method used. The structure of an alignmentmark can be a simple geometric structure or a set of geometricstructures.

For each alignment mark on a component, a reference mark can be added toa target platform for the component placement to refer to. In a specificembodiment, the present invention provides a solution for placing acomponent on a target substrate or platform, such as printed-circuitboard, mother board, ceramic board, bare die IC, or other packageddevice or component, by incorporating a set of one or more referencemarks at pre-determined regions on or within the component. Thereference marks can be connected to other reference marks on the sametarget platform to create a conduction path. A reference mark can beconnected to a ground point in a target platform, or it can simply be asurface marking on the target platform, depending upon the probingmethod used.

In preferred embodiments, the present invention provides probe solutionsfor improving the accuracy in component placement. The probe can be asingle-sensor probe or it can be an array of sensors. A single-sensorprobe can detect the accuracy of a component placement and the status ofa component's contact condition on a target platform. A probe composingof an array of sensors to become a multiple-sensor probe can adjust thedeviation of a displaced component by aligning the one or more alignmentmarks on the component to the probe position that has been aligned to areference mark. A multiple sensor probe can detect the deviation of acomponent and feed back the information to the placement equipment tofix the position deviation. While a single alignment mark can fix thecomponent displacement error, two alignment marks can fix the placementorientation error. The probe can be a resistance probe, a capacitanceprobe, an optical probe, or any combination thereof. The resistanceprobe is for on/off measurements, the capacitance probe is for measuringthe relative overlapping area between a sensor surface and a targetreference, and the optical probe is for measuring the reflection from atarget reference. The resistance probe is a contact probe. Thecapacitance probe and optical probe are non-contact probes. The range ofactive sensors on the multiple-sensor probe can be determinedautomatically by sensing the size of a target reference mark.

In a specific embodiment, the present invention provides a method foraligning a component on a target platform under various probingtechniques.

The method includes placing a component containing one or more alignmentmarks to a region on a target platform. The region on the targetplatform contains a target structure and a set of reference marks to bemonitored by a probe to determine whether or not the component hasaccurately been placed on the target structure. On a PCB, the targetstructure is a component land pattern. For each alignment mark on thecomponent, a corresponding reference mark can be included on the targetland pattern. The spatial relationship of the alignment marking on thetop to the contact array on the bottom of the component should match thespatial relationship of the reference marking with respect to thecontact array on the land pattern. The method also includes the way toalign the multiple-sensor probe to the target land pattern, to determinethe active range of the set of multiple sensors, and to adjust thecomponent position with respect to an aligned probe.

Since the spatial relationship of alignment marks to the associatedcontact array can be accurately controlled in the device fabrication orcomponent packaging process, a fixed spatial relationship between analignment mark and a contact array on a device or package can beensured. This allows a package with a minor obliquity in its physicaloutline (an oblique package) or a bare die not perfectly centered-cutalong the scribing line between adjacent dies (off-center-cut die) tostill be used for assembly. This is because in assembly the componentplacement can rely on the position of the alignment marks, rather thancounting on the geometrical tolerance of the package outline. In aspecific embodiment, the present invention provides a method formounting a physically out-of-specification device or package on aprospective contact region on a PCB or target platform. Many physicallyout-of-specification devices which previously would have caused amisalignment problem in assembly no longer need to be rejected. A largertolerance in the die cutting of device or in the molding of componentpackage means a higher component yield.

Many benefits can be achieved by way of the present invention overconventional techniques. The present technique provides an easy to useprocess that relies upon conventional technology. In some embodiments,the method provides a mean to improve yields, reduce rework, and enhanceplacement accuracy. Additionally, the method provides a process that iscompatible with conventional process technology without substantialmodifications to the conventional equipment and processes. The presentinvention is especially useful for the devices with ultra-fine contactpitch and contact count exceeding hundreds or even a thousand.

Moreover, the invention provides a method for monitoring the placementof devices packaged with anisotropic conductive elastomer (ACE) as aninterconnect interface. It enables the assembly of non-solder-ball basedpackaged devices onto a target platform or PCB effectively and preciselyfor a variety of applications. The anisotropic conductive elastomercontains a sea of tiny conducting metal tubes embedded in an elasticinsulating silicone membrane that conducts current only in a certaindirection. It has been used as an interconnect in high density and highpin count test sockets to offer excellent contact, repeatability, andhigh frequency characteristics in IC device tests. It is feasible to beused as the interface interconnect for devices and packages. A device orcomponent laminated with ACE can be directly mounted onto a targetsubstrate. A clamp shell can be used to hold the devices togetherwithout the need of soldering the devices onto a target substrate if anaccurate placement can be achieved. The alignment technique can make theassembly of ACE laminated devices on electronics system feasible.

Depending upon the embodiment, one or more of these benefits can beachieved and will be described in more detail throughout the presentspecification. Various additional objects, features and advantages ofthe present invention can be more fully appreciated with reference tothe detailed description and accompanying drawings that follow.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is an example of an ideal package.

FIG. 1B is an example of an oblique package.

FIG. 1C is a different example of an out-of-specification package with acontact array off-grid.

FIG. 2 shows the placement of an off-grid component on a land pattern.All contacts on the component are shifted by a same offset from the landpattern after placement.

FIG. 3 illustrates the placement of a component with a set of conductionpaths on a target platform with a matching land pattern and referencemarks for checking the placement accuracy and contact condition.

FIG. 4 shows a different configuration of an alignment mark, where alltest-accessible points are on the same side as the contact array. It isa simplified diagram of an indirect alignment path and an indirectalignment mark.

FIG. 5 shows an example of using a single-detection-point probe to checkfor the placement accuracy and the component contact condition.

FIG. 6 shows an example of using a 4-point probe for detecting theposition deviation in a component placement.

FIG. 7 shows a generalized test probe consisting of an array of testpoints for detecting the deviation in a component placement. The activetest points on the probe can be determined on-the-fly.

FIG. 8 shows an example on the use of a set of SR latches to track thepositions of active test points and to align the center of a test probeto the center of a target reference mark. The set of SR latches and canbe replaced by a set of latches with write enable.

FIG. 9 illustrates the use of two sets of latches to determine thedirection and displacement of a deviated component. One set of latcheswith status being shown in the first digit of a binary pair indicatesthe overlap status between an alignment mark and a reference mark. Theother set of latches with status being shown in the second digit of abinary pair indicates the position of active test points.

FIG. 10 illustrates the use of three sets of latches to align a smallalignment mark to the center of a large reference mark. The third digitin the tuple indicates the effective range of active test points for asmaller alignment mark to align to.

FIG. 11 shows a graphical interpretation on the use of a generalizedprobe to fix a deviated component, where the size of an alignment markon a component is smaller than that of a reference mark.

FIG. 12 is a simplified diagram showing a test probe laminated with ACEmaterial at an interconnect interface to improve the contact conditionbetween the test points and the alignment mark or between the testpoints and the reference mark.

FIG. 13 shows that incorporating two alignment marks on a component canfix the orientation error in a component placement.

FIG. 14 shows a simplified diagram of a serial alignment chain on atarget platform.

FIG. 15 is a simplified diagram of a capacitance sensor, which uses aguard to focus the sensor's electrical field.

FIG. 16 is a simplified diagram showing a set of capacitance sensorsgrouped in an array to form a multiple-sensor capacitance probe for usein non-contact electrical aligning.

FIG. 17 in an example of the use of an electrical alignment technique tomonitor component stacking

FIG. 18 shows a different application of an electrical alignmenttechnique for aligning an object with a quad-sensor alignment mark ontoa second object with a quad-triangle reference mark.

FIG. 19 shows how a transparent optical path on a component can be usedas an optical alignment mark. If coupled with a matching a reflectivepad on a target platform, it can be used to improve the componentplacement accuracy.

FIG. 20 is a simplified diagram showing an array of photo-detectorsgrouped in an array to form a multiple-detector optical probe for use incomponent alignment.

FIG. 21 is a simplified diagram of an optical setup with fourphoto-detectors and four I/V amplifiers for use in adjusting thedeviation of displaced components.

DETAILED DESCRIPTION OF THE INVENTION

According to the present invention, alignment and probing techniques toimprove the component placement accuracy during assembly are described.More particularly, the invention includes methods and structures todetect and improve the component placement accuracy on a target platformby incorporating alignment marks on a component and reference marks on atarget platform with various probing techniques. A set of sensorsgrouped in an array to form a multiple-sensor probe can detect thedeviation of displaced components in assembly. Merely by way of example,the invention can be applied to placing packaged semiconductor devicesonto electronic substrates for the manufacture of electronic systems.But it would be recognized that the invention has a much broader rangeof applicability. Further details of the present invention can be foundthroughout the present specification and more particularly below.

Alignment Mark

According to preferred embodiments, an alignment mark is a referenceregion on a component that an external probe can use it to monitor theaccuracy of component placement on a target platform. The alignment markincorporated on the component can be a conduction path connecting a topsurface area to a bottom surface area on the component as a directalignment mark, or it can be a different conduction path connecting twosurface areas on the bottom of the component as an indirect alignmentmark. Although the top-down conduction path does not need to be astraight path, with a straight path from top to bottom or a matching topand bottom location on the component it is easier to visualize theposition of the bottom contact point associated with the alignment mark.The surface area of the conduction path on the bottom can be either thecontact point on the contact array or a different access point. Besidesbeing a conduction path within the component, the alignment mark canalso be a simple surface marking on a component, depending upon theprobing method used. The structure of an alignment mark can be a simplegeometric structure or a set of geometric structures.

An alignment mark on a component can be a single alignment mark or a setof alignment marks. Normally, one alignment mark is adequate. A secondalignment mark is added to the component if the component size is largeor may require an orientation adjustment to improve the placementaccuracy. The tangential deviation from the ideal land pattern forfar-end contacts in a large component could be significant if there isan orientation error. For a small component, the impact due to a minororientation error is insignificant.

Depending upon the probing techniques, a probe can be either in directcontact or not in direct contact with the alignment mark when it is usedto monitor the component placement accuracy. If a resistance probe isused to monitor the position of an alignment mark, it requires a directcontact. If a capacitance probe is used to monitor the position ofalignment mark, it does not require a direct contact. In both cases, thealignment mark is an electrical alignment mark because there is aconduction current flowing through the alignment mark, except one is aDC current and the other is an AC current. If an optical probe is usedto monitor the reflection from the surface of an alignment mark, thenthe alignment mark is an optical alignment mark. Thus, a reflective,conductive marking on the top surface of a component can be used as anelectrical alignment mark or as an optical alignment mark depending uponhow the alignment mark is constructed and which probing technique isused.

Reference Mark

For each alignment mark on a component, a corresponding reference markcan be added to a target platform for component placement to refer to.The spatial relationship of the alignment mark to the contact array onthe component should match the spatial relationship of the referencemark to the associated land pattern on the target platform. Thereference mark can be part of the component contact land pattern on thetarget platform. Depending upon the probing technique and theapplication requirements, the reference mark may or may not need to makea direct contact with the bottom of the alignment mark. In a specialcase, if the reference mark is simply a surface marking on a targetplatform and if the alignment marking is also a simple surface marking,then the top position of the alignment marking on the component can bechosen in such way that it matches the center point or one of the cornerpoints of a contact array on the land pattern to eliminate the need foradditional reference marks on the target platform.

Uncertainty in Component Placement

One useful aspect of the herein described alignment techniques is thatthe placement accuracy no longer relies on the component's physicaloutline, nor does it rely on the grid accuracy of a contact arrayrelative to the edges of a component. For example, if the moldingprocess causes a minor obliquity in the finished package such that thecontact array on the package is not perfectly in parallel with the edgeof the package (an oblique package), it is difficult to place thepackage correctly on a land pattern under conventional technology,especially when the package size is large and the array pitch size issmall; nor for a package with its contact-array shifted from the idealgrid location (an off-grid package). Conventional placement equipmentuses the Cartesian coordinates at the center of the land pattern as areference point to place a component on it, based on the assumption thatthe component's physical outline is perfect and its contact array isprecisely located relative to the edges of the component per packagemechanical specifications.

FIG. 1A shows an example of an ideal package. It has a perfect physicaloutline and all package contacts are on prescribed grid locations. FIG.1B shows an example of an oblique package, where the contact array isnot in parallel with the package physical outline and is tilted from theideal grid location as viewed from the package outline. FIG. 1C shows adifferent example of an out-of-specification package where the contactarray is not properly center-molded and thus bears an offset from idealgrid location as viewed from the package outline. The packages in FIG.1B and FIG. 1C are out of physical specification and are difficult touse in conventional surface mount assembly. The imperfections in thedrawings are exaggerated to illustrate the concept.

FIG. 2 shows an example of placing an off-grid component 200 on a targetland pattern 208. As shown, the contact array 209 on component 200suffers an offset 207 in the lower-left direction. All contacts oncomponent 200 will have the same offset 207 from the target land pattern208 after placement. This is different from the placement of an obliquepackage where the skew or offset at the corner contacts is larger thanthat at center.

Adding alignment marks to a component could significantly reduce theadverse impact due to the imperfection in the package outline andeliminate the contact position uncertainty caused by the deviation of acontact array from its ideal grid location, even if the package outlineis still in spec. The alignment mark can be placed within or beyond thecomponent's contact array. It is fabricated at a predetermined spatialrelationship with respect to the component's contact array, rather thanat a location based on the component physical outline or the distancefrom the edge of package.

The alignment mark decouples the equipment's reliance on the idealphysical outline of components. Thus, some physicallyout-of-specification components that were scraped in production before,such as packages with a minor obliquity in physical outline, packageswith a contact array deviated from an ideal grid location, or bare diesnot perfectly centered cut along the scribing line between adjacentdies, could become useable for system assembly. Further detail of thestructure and the operation of the alignment mark are described asfollows. Certain methods and variations are also provided throughout thepresent specification.

Component Alignment Using a Resistance Probe

In this embodiment, an alignment mark is an electrical conduction pathincorporated in a component or package at a pre-determined spatiallocation with respect to its contact array. The conduction path can beconstructed from the top surface of a component, where a test signal canbe applied, to the bottom surface of the component, where a matchingcontact or reference pad on a target platform can be connectedunderneath. The conduction path on the component and the matchingreference pad on the target platform are paired to monitor the accuracyof the component placement. If a component were accurately placed on atarget platform, the matching reference pad would appear underneath thebottom surface of the conduction path and a conduction current would bedetected when a voltage were applied to the top surface of thecomponent.

FIG. 3 shows an example of placing a component 300 on a target platform310. The component 300 is an encapsulated package 301 with a pluralityof external connections at contact array 309 and one or more conductionpaths 302, 303 as alignment marks. On the target platform, a landpattern 308 and a set of reference pads 306, 307 are pre-fabricated. Thereference pads 306, 307 would be underneath the bottom surface of theconduction paths 302, 303 if the component 300 were aligned to a landpattern 308. This diagram is merely an example, which should not undulylimit the scope of the claims herein. One of ordinary skill in the artwould recognize many variations, modifications, and alternatives, suchas the component can be a bare die with an embedded conduction path fromtop to bottom as an alignment mark, an integrated circuit encapsulatedin a plastic package or in a ceramic material package with solder bumps,or a device laminated with a layer of an ACE membrane for externalinterconnects but without solder bumps, or any combination of these, andthe like.

The conduction path in component 300 can be a straight pipe or anyirregular shape of conduction trace 303 running from top to bottom inthe component. One or more conduction paths can be incorporated in thecomponent. The external accessible points 304, 305 associated with theconduction paths 302, 303 can be a simple circular pad. Other padshapes, such as square, rectangular, triangle, trapezoid, or acombination of these, may be used. A resistance probe is placed on theexternal access points 304, 305 to monitor the status of the placementaccuracy. The conduction path can be a metal, a doped semiconductorpath, or other detectable entity.

In FIG. 3, a set of reference pads 306, 307 is pre-fabricated on thetarget platform 310 as a target reference to guide the componentplacement. The reference pad can be a simple circular pad, althoughother shapes, such as a square, a rectangular, a triangle, a trapezoid,a set of ground dots, or a combination of these, may also be used. Thereference pads 306, 307 on target platform 310 are named the “referencemarks”. For an alignment mark on a component, a correspondent referencemark can be added to the target platform. The location of the referencemark on the target platform 310 is pre-fabricated in such a way that thespatial relationship between the reference marks 306, 307 and the landpattern 308 on the target platform matches the spatial relationshipbetween the bottom contact of alignment marks 302, 303 and the contactarray 309 on component 300.

The size of a reference mark is determined by the variance of theplacement system, which includes such factors as the imperfections in acomponent's outline or contact array, imperfections in a targetplatform, in the equipment's precision, effects of mechanical aging,database rounding errors, etc. A system with larger cumulative placementuncertainty requires a larger reference mark. The size of the referencemark should be large enough that at each initial component placement thealignment mark will be at least partially within the boundary of thetarget reference mark. The initial component placement is the placementbased on the coordinates of the reference mark or target land patternstored in the placement database. In addition, the size of the alignmentmark is preferably the same as or smaller than the size of referencemark, since it can impact the accuracy of the component placement, aswill be explained later.

Besides a top-down structure, where one test point associated with theconduction path is at the top surface of component, it is also possibleto have both test points 411, 412 on the same side as the contact array409, as shown in FIG. 4. Two additional traces 406, 407 are added to thetarget platform 410 in this case to connect the test points 411, 412associated with the conduction path to the external accessible points403, 404 on the target platform 410. The power and ground tips of aresistance probe can be applied to the two externally accessible points403, 404 to detect if there is a conduction current to determine if thecomponent has been correctly placed on the target land pattern 408. Theconduction path with both test points located at the same side as thecontact array is called an indirect alignment path. Its contact pointsare the indirect alignment marks.

If several indirect alignment paths on different components areconnected into a serial daisy chain on a target platform, it can be usedto monitor the contact condition of all of the components in the chainor to check if all of the components are still in place when the systemis in use. This is useful for electronic systems assembled withsolderless components, such as components using ACE as the interconnectinterface, where the indirect alignment paths for all ACE basedcomponents can be connected in a single alignment chain or in severalshorter chains. A supply voltage and detectors, such as LED diodes, canbe connected to each chain to monitor the connection status of allsolderless components in the chain.

Incorporating alignment marks on components not only can detect theaccuracy of a component placement on a target platform, but also can fixthe component position deviation after placement. However, it is theconfiguration of a test probe and the handling of signals detected bythat test probe that determine the functions of the alignment mark.While a single-test-point probe, namely a single-sensor probe, candetect the placement status and the interface contact condition, amultiple-test-point probe, namely a multiple-sensor probe, can detectthe direction and the magnitude of a deviation in the componentplacement, as is explained later herein.

FIG. 5 shows an example of using a single-test-point resistance probe520, to monitor the alignment status of a component 500 on a targetplatform 510. Assume a component 500 containing two alignment marks 501and 502 has aligned to a land pattern on target platform 510, wherethere is a set of corresponding reference marks 503 and 504. Aconduction path is formed from the supply tip 521 of the probe 520,through the alignment mark 501, to the reference mark 503 on the targetplatform 510. A second conduction path is from the alignment mark 502 tothe reference mark 504 on the target platform 510.

To avoid the two reference marks 503, 504 from floating at test, bothshould be connected to an internal plane 507 on the target platform 510,which is connected to a known reference voltage or ground 508 through asurface contact 505. A different approach is to connect the tworeference marks together internally. A probe can be applied to test theconnectivity after a component is placed. For example, if the supply tip521 of probe 520 is applied to the top surface of the alignment mark 501and its ground tip 515 is applied to the top surface of the alignmentmark 502, then a closed current loop is formed to detect a conductioncurrent from the probe's test tip 521, through the alignment mark 501,the reference mark 503, the internal trace or plane 507, back to thereference mark 504, the alignment mark 502, and finally to the probeground tip 515, if the component 500 is well aligned on the targetplatform 510. Although two alignment marks are shown, one alignment markis adequate if the orientation error is less likely in the componentplacement.

Alternatively, if the reference pads 503, 504 are connected to aninternal plane 507 and a reference voltage or ground 508 is applied tothe surface contact 505, then the reference pads 503, 504 will be at thereference voltage or ground. As the test tip 521 is applied to the topsurface of any alignment mark on the component 500 in this case, aclosed loop is formed from the probe, through the alignment mark, thecorresponding reference mark, to the system ground and then to the probeground, provided that the two grounds are connected together.

As shown in FIG. 5, a test probe 520 may contain a resistor 522, alight-emitting diode 523, a sound buzzer, or a current meter 524 toindicate the status of the placement. The resistor limits the maximumcurrent to flow through the probe and the alignment mark. Thelight-emitting diode provides a visible signal to indicate if it is anaccurate placement. The sound buzzer enables users to hear. The currentmeter shows the amount of current through the probe. The probe can bebuilt into the placement equipment for monitoring the placement status.It can also be a probe for checking the component placement accuracy andcontact condition by users manually.

FIG. 6 illustrates the use of a 4-point probe to improve the accuracy ofcomponent placement. A multiple-detection point probe is essential tofix the component deviation in placement. A 4-point resistance-probewith test points A, B, C, and D is used in this example. A circularreference mark 602 is shown in a large dot circle. An alignment mark 601also in a circular shape is shown in a large solid circle. To make theillustration simpler, assume that the size of the electrical alignmentmark 601 is the same as the size of the reference mark 602, and alsoassume that the probe used in this placement has all four test pointsmatch the circumference of the reference mark 602. If a square referencemark is chosen as an example, then all four test points on the probeshould match the four corners of the square reference mark after theprobe position is aligned with the reference mark.

To fix the deviation in component placement, first the probe positionneeds to be aligned with respect to the reference mark by moving allfour test-points A, B, C, and D on the probe onto or adjacent to theboundary of the target reference mark 602. Then, a component is moved inand placed at the coordinate of a target land pattern per theinformation stored in a placement database. If there were a deviation incomponent placement, the alignment mark 601 on the component would onlypartially overlap with the target reference mark 602 on the targetplatform. The 4-point resistance probe can detect this partialoverlapping. For example, assume the deviation is toward the lower, leftcorner and only the test point C at the lower, left corner is in contactwith the alignment mark 601. Then, only the test point C detects acurrent flow through the alignment mark 601 to the ground reference mark602. There is no current being detected by the test points A, B, and D.This is because they are in a region outside the alignment mark 601,which is not electrically conductive. Using this test result as afeedback, the placement equipment can thus know the deviation of thecomponent placement. In this example, shifting of the component positionupward and right-ward can fix the placement deviation, i.e., from thedirection where current is detected through the alignment mark 601 bytest points to the direction where no current is detected. This processcontinues until the alignment mark 601 on the component is moved to anew position in contact with all of the test points. Then, the componentis accurately positioned.

Consider the situation where, after the component placement, thealignment mark 601 is at a location such that two test points detectconduction current, say, for example, the two left test points. Then thealignment mark, and the component, should be shifted rightward until itreaches a position such that all four test-points can detect the currentflow. In this case, the test probe, the reference mark and the alignmentmark are all aligned and the component is accurately placed. The testprobe and a component's pickup header in the placement equipment canboth be adjusted independently.

Adjusting the test probe in line with the target reference mark at thebeginning of each component placement is essential to ensure theplacement accuracy. In this special case it can be done without acomponent between the probe and reference mark by making the probecontact the reference mark. The deviation from the center of the probeto the center of the target reference mark can be determined bymonitoring which test points conduct current. By shifting the probeposition toward the direction where no current is detected by the testpoints, the probe can be aligned with the target reference mark. This isa probe alignment step.

A multiple test point probe has more capability than a single test pointprobe in monitoring the component placement. While a single-test-pointresistance probe can be used to detect the placement accuracy and thecontact condition, a multiple-test-point probe can be used to detect andprovide feedback on the deviation of a displaced component.

FIG. 7 shows a generalized test probe consisting of an array oftest-points 701. If the area detectable by the array of test points 701is larger than the size of a reference mark 700, then only a subset oftest points in the array will be activated to detect the positiondeviation. The subset of test points to be activated in each componentplacement can be determined automatically on-the-fly in a similarmanner. This can be done by lowering the test probe—assume it is aresistance probe—so that it is in contact with the reference mark 700and applying a voltage to all test points in the array 701 to see whichtest points conduct current.

If all test points conducting current were the inner test points in thearray, then the inner test points detecting current 702 could be chosento facilitate the component position adjustment. Those test points inthe array not conducting current flow 703 could be ignored in thecurrent component placement. The subset of inner test points in thearray being activated to facilitate the component position adjustment isthe active test points 702. Those test points being ignored in theposition adjustment are the inactive test points 703.

Since the center of a multiple-test-point probe is known from the probestructure, the probe design can be simpler if the set of active testpoints can be repositioned to center around the center of themultiple-test-point probe. This can be done by moving the probe to a newposition until all inner active test points are in the central region ofa multiple-test-point probe. In this new position the center of the testprobe is aligned with the center of the target reference mark and withthe outer range of the active test points matching the boundary of thereference mark.

If part of the test points conducting current were at the edge of themultiple-test-point array, then either the size of the probe is toosmall or the probe position is not lined up with the reference mark.Assume the size of probe is too small, i.e., smaller than the size ofthe target reference mark. In this case there could be an uncertainty inthe component position adjustment due to being unable to align the smallprobe to the center of the reference mark. Preferably, the range of testprobe is compatible with or is larger than the size of reference mark.And if the probe position is not lined up with the reference mark, thenit should be adjusted until the center of the active test points in thearray are lined up with the center of the reference mark.

A set of asynchronous set-reset (SR) latches can be implemented in theelectronics of a test probe, one SR latch for each test point on theprobe. Corresponding to the subset of test points on the probe which areactivated to monitor the component placement, a subset of SR latches isset to indicate the positions of the active test points for eachcomponent placement. The entire set of SR latches is cleared at thestart of each component placement. A latch with write-enable can also beused to implement the function of a SR latch.

FIG. 8 illustrates a method of using the set of SR latches for aligninga test probe to the center of a target reference mark. To align amultiple-test-point resistance probe to a reference mark, after making acontact with the reference mark, the SR latch is set if thecorresponding test-point conducts current and the SR latch remains resetif the corresponding test-point detects no current. Counters can be usedto scan all rows and columns on the set of SR latches to find out whichrow and which column detect the highest number of ones. The interceptionof the highest row and column is the center of reference mark. A rowcounter, a column counter, a comparator, registers for tracking the rownumber and the column number that have the highest number of ones, and aset of multiplexers to select proper inputs to these logic, etc., can beused to achieve such a function. This is merely an example, which shouldnot unduly limit the scope of the invention. By comparing the row numberand the column number that have the highest number of ones with the rownumber and the column number at the center of probe, the displacement toalign the probe to the center of reference mark can be determined. Theprobe can thus be shifted accordingly to align with the center of thereference mark. After the re-positioning of the probe, the SR latchesare cleared to record the new conduction status for all test points onthe probe. The SR latches being set at this stage contain theinformation of the set of active test points aligned with the center ofthe reference mark.

For aligning a component, another set of transparent latches can beadded to the probe electronics to track the instantaneous conductionstatus of all test points each time the component changes to a newposition. The contents of transparent latches record the overlap statusbetween the alignment mark and the reference mark. Their contents willbe compared with the contents of the SR latches. If the contents of bothsets match, then the component is accurately aligned on the target landpattern.

FIG. 9 shows an example of using two sets of latches to derive thedirection and displacement of a deviated component. The first digit inthe binary pair is the status of the set of transparent latches, whichindicates the overlap status between the alignment mark and thereference mark. The second digit in the binary pair is the status of theset of SR latches, which indicates the position of the active testpoints.

By comparing the contents of these two latches, or the value of thebinary pair, the direction and the adjustment displacement for amisplaced component can be derived. To make the explanation simpler, byway of example, assume the size of the alignment mark is the same as thesize of the reference mark. A “1” in the first digit indicates the areawhere the alignment mark overlaps with the reference mark. A “1” in thesecond digit indicates the location of active test points. By referringto the “11” region in FIG. 9, which is known by a simple AND logic,there are three columns to the right side containing all “01”s and onlytwo rows to the up side containing all “01”s inside the reference markrange. The placement equipment thus knows that a shifting of threepositions to the right side and two positions to the up side canre-position the component correctly. The position of test pointscontaining “01” can be known by a simple exclusive OR gate. Thisillustration is merely an example, which should not unduly limit thescope of the invention.

While a larger reference mark can extend the range of adjustment for adeviated component, a smaller alignment mark can still work properly ifthe test probe is large enough, i.e., if the range of all test points onthe probe is larger than the size of the reference mark. To support asmaller alignment mark, a third set of latches can be incorporated inthe probe electronics to indicate the effective sub-range of active testpoints where the small alignment mark will be referred to as being inalignment. The third set of latches servers as an enable mask indetermining the effective range of “01”s in the derivation of thecomponent shift displacement.

In FIG. 10, the third digit in a binary tuple indicates the sub-rangeinside a reference mark that matches the size of an alignment mark. A“1” in the third digit, such as the binary tuples “111” and “011”,indicates the location of the sub-set of active test points for use asthe target for a smaller alignment mark to align to. While the entirerange of active test points, determined by the size of the referencemark, is used to detect the presence of the alignment mark, it is thesub-range of active test points being used as the target for the smallalignment mark to align to. A “11” in the first two digits of the tuple,i.e., “11.times.”, as shown in the example of FIG. 10, represents theoverlapping area between the alignment mark and the reference mark. Thetuple “011” indicates the area where the alignment mark is still not inline with the reference mark. There are two columns to the right side of“111” containing “011”s and two rows to the top side of “111” containing“011”s in the target sub-range in FIG. 10. Thus, a shifting of thecomponent two positions upward and two positions rightward can stillfully align the alignment mark to the reference mark, even if thealignment mark is smaller than the reference mark.

The actual sub-range of active test points associated with a smalleralignment mark can be derived by referring to the diameter informationof the alignment mark stored in the placement database or by using animage sensor associated with the placement equipment to measure thediameter of the alignment mark on-the-fly.

FIG. 11 shows a graphical interpretation on the use of a generalizedtest probe to fix the position of a deviated component. An alignmentmark 1101, which reflects the position of a component, is smaller than areference mark 1102 in this example. Before each component placementstarts, the range of active test points on the probe should bedetermined in advance, i.e., by moving the probe on top of the referencemark 1102, without a component between the probe and the reference mark,and applying a voltage to all test points to see which test pointsconducts current. Since the center of the test probe is known, the setof active test points can be chosen to have the probe in line with thecenter of the reference mark 1102 and to have the outmost active testpoints adjacent to the boundary of reference mark 1102. In FIG. 11, theinactive test points on the probe are shown as dot circles and theactive test points are shown as solid circles and as a solid black dot.The sub-range of active test points to be used for aligning a smalleralignment mark to the reference mark can also be derived on-the-fly byusing an image sensor inside the placement equipment to measure thediameter of the alignment mark or by referring to the diameterinformation of the alignment mark stored in a placement database.

After initial component placement, the alignment mark 1101 on thecomponent is either entirely or partially within the range of the activetest points being defined by the reference mark 1102. Assume there is adeviation in the initial component placement and assume only one corneractive test point 1105, shown in the solid black dot in FIG. 11, detectscurrent flow. The placement equipment could then use the subset of theactive test points as a target to shift the alignment mark rightward andupward until the alignment mark matches the entire subset of active testpoints 1106, shown as the big, central dot circle in FIG. 11. Thus, thecomponent can still be accurately aligned to a target land pattern evenif the alignment mark is smaller than the reference mark.

On the contrary, if the area of the alignment mark were much larger thanthe area of the reference mark, the placement accuracy would bedegraded. This is because the range of active test points defined by asmall reference mark is rather limited. It would be unable to proceedwith the component adjustment process because no adequate feedback wouldbe available to instruct the large alignment mark on the component tomove its center to line up with a limited set of active test points. Forresistance probing, the effect of using a larger alignment mark issimilar to the effect of using a single-test-point probe, where the areacovered by a single test point probe is very narrow, similar to theeffect of a smaller test probe or a smaller reference mark.Nevertheless, a smaller reference mark can detect much higher placementaccuracy under a single-test-point probe, unless the size of thereference mark is too small to be detectable.

If, after an initial placement, the component were placed beyond thereference mark, then no o conduction current would be detected by any ofthe active test points. This would indicate that either the placementequipment was improperly set up, or that the worst-case deviation in thecomponent placement was under-estimated, causing the alignment mark tofall beyond the under-sized reference mark. An improper system setup oroperational error can thus be detected easily.

To improve the durability of a test probe and to ensure good contact toan alignment mark and a reference mark, an anisotropic conductiveelastomer (ACE) 1205 can be laminated on the surface of a generalizedresistance probe 1200 as the interconnect interface, shown in FIG. 12.In this figure, each test point consists of a conduction pipe 1202inside the probe 1200. This figure is merely an example, which shouldnot unduly limit the scope of the invention. For example, the probe isshown in a circular shape, but other probe shapes, such as a square, arectangular, a triangle, a trapezoid, other irregular shapes, or even aflexible cable, is likely. Also, although the test points are shown in aregular array in the figure, the test points in array can be arranged inany configuration based on the requirements of an application.

To resolve the orientation error in a component placement, two alignmentmarks can be incorporated on the opposite sides of the component. It ismore likely to encounter an orientation error for components with alarge contact array or an ultra-fine contact pitch.

FIG. 13 shows an example of incorporating two alignment marks on acomponent to fix the orientation error in component placement. Assumethat the component is tilted counter-clockwise after initial placementand assume that the size of an alignment mark is the same as the size ofa reference mark. Two large solid circles in the diagram represent twoalignment marks 1301, 1302 on a component. An artificial solid linelinking these two-alignment marks 1301, 1302 is drawn to show animaginary orientation of the component after initial placement. Twocorresponding reference marks 1303, 1304 on a target platform are shownin large dot circles, connected by an artificial dotted line to indicatethe ideal target orientation. The acute angle intercepted by the solidline and the dot line is the orientation error after the initialplacement. Assuming that there is no displacement error, theinterception point is the center of the component 1300. Two test probesmay be used to monitor the placement of a component containing twoalignment marks, although one is adequate if the active test points andthe center of the test probe can be recorded by the placement equipmentfor each probing event.

Before placing a component, the probes must be aligned with the targetreference marks. For a resistance probe, this is done by placing theprobe in contact the reference mark to determine the range and thecoordinates of the active test points with respect to the center of thereference mark. Then, a component is picked and placed at the targetland pattern with the probe moved out of the way. If there is noorientation error in the component placement, both alignment marks 1301,1302 will stay exactly on top of the reference marks 1303, 1304 andmatch the range of the active test points. If there is an orientationerror, then some of the active test points on the probe will be outsidethe range of the alignment mark and conduct no current. By monitoringthe conduction status of the active test points, the direction of therequired adjustment can be determined, i.e., by rotating the componentor alignment mark toward the side where the active test points detect nocurrent, provided that the component movement assembly is capable ofperforming angular position adjustment.

In FIG. 13, the two alignment marks 1301, 1302 on the component are inthe upper right and lower left locations. If a counter-clockwiseorientation error occurs after component placement, then the active testpoints 1305 detecting conduction current will lean to the left-hand sidefor the upper probing and the active test points 1307 detectingconduction for the lower probing will lean to the right-hand side. Byrotating the component from the side that active test points detectcurrent to the side detecting no current, i.e., in the direction ofclockwise rotation, the component orientation error can be fixed. If allcurrent sensing tests points were to appear at the same side under bothprobing, say all on the left-hand side, then a displacement error hadtaken place and the component should be shifted right-toward to fix itsdisplacement error.

The alignment technique can be adaptive to application needs. Forexample, if the placement system has intrinsic skew such that theplacement error is larger in one direction than the other, then arectangular reference mark can be chosen with its long sidecorresponding to the direction of the larger deviation. Preferably, thesize of the probe is larger than the size of the reference mark in orderto align the probe and to determine the range of the active test pointson the probe automatically. Also preferably, the size of the referencemark is larger than or is about the same size as the alignment mark.

Resistance probing that requires direct contact measurements may causecontamination of solder paste around a target land pattern in situationswhen the displaced component needs to be moved around on the targetplatform. There is no such problem for the components laminated with ACEfor use as an interconnect interface. The components laminated with anACE interconnect interface also make good contact with the land patternon the target platform.

To avoid the adverse effect of potential solder paste contaminations, itis also possible to laminate a thin layer of solder material on thesurface of land patterns on PCB, similar to the solder ball material ona package, during PCB fabrication. Then, the solder paste-printing stepcan be eliminated in the surface-mount assembly, thus preventing solderpaste contaminations.

A top-down alignment mark and indirect electrical alignment paths can becombined and incorporated within a same component. The top-downalignment mark can improve the placement accuracy and the indirectalignment paths can be linked in a serial alignment chain to monitor thecontact status of all components in the chain, as shown in FIG. 14. Thisis particularly useful for the electronic systems assembled withcomponents laminated with ACE interconnect.

Alignment Using Capacitance Probe

The electrical alignment can also be achieved by using a non-contactmethod. Rather than using a resistance probe to measure the on/offresistance at the direct contact point, a capacitor probe can be used toalign a component without touching or contacting it.

FIG. 15 shows an example of a capacitance sensor manufactured by LionPrecision of St. Paul, Minn. To improve measurement accuracy, theelectrical field from the sensor needs to be confined within the spacebetween the sensor's surface and the reference target. A separateconductor kept at the same voltage as the sensor itself is used as aguard to surround the sides and the back of the sensor. When an ACsignal is applied to the sensor, a separate circuit applies the exactsame excitation voltage to the guard. Because there is no voltagedifference between the sensor and the guard, there is no electricalfield between them. Any other conductors beside or behind thecapacitance probe form an electrical field with the guard, instead ofthe sensor. Only the unguarded front end of the sensor forms anelectrical field with a reference target. The sensor creates anelectrical field that is approximately a projection of its size andshape. For instance, a round sensor could project a cylindricalelectrical field to a reference target. In Lion Precision's capacitorsensor, the cylindrical electrical field could spread up to 30% at aneffective range of 40% of the sensor diameter.

As an AC signal is applied to the sensor surface, AC current will flowthrough the capacitance formed in the gap between the sensor and thetarget reference. The amount of current that flows is dependent on theamount of capacitance between the sensor and the target reference, i.e.,the current is determined by the size of separation and the extent ofthe overlap between the surfaces. If the sensor is fully aligned withthe reference target, the capacitance formed in the gap is the largestand the AC current will be the highest. If the sensor is entirely beyondthe reference target, there is no surface area overlap and thecapacitance is irrelevant, provided that no foreign conductor besidesthe reference target is within the sensor range.

A set of guarded capacitance sensors can be grouped in an array to forma multiple-sensor capacitance probe, as shown in FIG. 16, fornon-contact electrical probing. The sensors in the array can also beselectively activated as required. For example, FIG. 16 shows anine-sensor probe, but if only the sensors at the four corners of theprobe are activated for placement detection, then it becomes afour-sensor capacitance probe.

For probing alignment, the exact capacitance detected by each sensor ona probe is not a major concern. This is because in alignmentapplications the purpose of the capacitance sensor is not to measure theexact separation between two parallel objects, where a precisecapacitance reading is essential. Rather, it is the relativecapacitances, determined by the extent of the overlapping between theprobe surface and the target reference area, which plays a key role. Theeffective range of a multiple-sensor capacitance probe for measuring theplacement accuracy is greater than that of a single sensor, providedthat no foreign conductor is within the range of the sensors' electricalfield to disturb the relative capacitance measurement.

As an AC excitation is applied to all active sensors on a probe, allsensors will detect a same capacitance between the sensor and the targetreference and thus will measure the same amount of AC current, providedthe probe is fully aligned with the reference target. If misalignmentexists, the portion of the active sensors on the probe detecting areaoverlap with the reference target will conduct the highest current whilethe portion of the active sensors beyond the reference target willconduct a minimal current. By comparing the relative magnitude of the ACcurrents among the array of active sensors, the direction of thedeviation can be known. The deviation can be fixed by aligning thecomponent alignment mark under the capacitance probe or by moving thedisplaced component toward the direction where the active sensors detectlower levels of AC current. The probe does not need to be in contactwith the target platform or the component in this process.

A capacitance sensor is normally calibrated to a grounded target. Thebody or outer jacket of the capacitance probe should be electricallywired to ground to improve the probe accuracy. The reference target alsorequires proper grounding. A reference target which is not properlygrounded could degrade the probe's sensitivity and accuracy.Fortunately, many targets, while not directly grounded, have asignificant capacitance to ground though their environment, such as thecomponent pickup header. The large capacitance eventually shorts thetarget reference to ground under AC excitation. Thus, connecting thereference mark on a target platform to a ground plane or connecting thealignment mark on a component's surface to its local ground is helpfulbut not essential as long as the ratio between the target's capacitanceto ground and the target's capacitance to sensor is reasonably large. Aratio over 10 will result in an error in capacitance measurement of lessthan 10%, as seen in the case of two capacitances in series.

When using a capacitance probe to align a component, only the surfacemarking on the top of the component is relevant. The surface area of thealignment mark on top of the component determines the capacitance valuedetected by a capacitance sensor. An electrical conduction path from thetop to the bottom of the component as an electrical alignment mark, asis used in the case of resistance probing, is no longer required.However, the Cartesian coordinate relationship between the alignmentmarking on the top and the contact array on the bottom of the componentshould match the coordinate relationship between the reference mark andthe target land pattern on the target platform. Alignment of analignment marking using a capacitance probe can be done by moving thealignment marking around under the capacitance probe, assuming the probehas already been aligned relative to a reference mark, and monitoringwhere all active sensors over the alignment marking detect a same amountof current which exceeds a certain minimum threshold. After thealignment marking is in line with the probe, the aligned component canbe lowered down to be placed on atarget land pattern accurately.

Using a non-contact probing, such as capacitance probing, aligning aprobe to a target reference and aligning a alignment marking to theprobe can be done independently. Thus, the size relationship between thealignment marking on the component and the reference mark on the targetplatform is no longer as critical as in the case of contact probing,provided that the probe detection range is large enough to cover both ofthem. Thus, a component with a large alignment mark can still be alignedaccurately on a smaller reference mark by using a non-contact probing.

The electrical aligning technique can also be used to monitor theaccuracy in the stacking of packaged chips, where a conduction path canbe inserted from the top to the bottom of the packaged chip as anelectrical alignment mark. The electrical alignment technique can alsobe used to monitor the accuracy in bare-die stacking, where a conductionpath from the top to the bottom of a bare-die can be created using ionimplantation, diffusion, or other methods.

FIG. 17 shows an example of using an electrical alignment technique tomonitor the stacking of components. Several variations are shown in theexample to illustrate the applications under different situations. Theaccuracy of component stacking can be known by directly placing a probeat the top of the top component alignment mark and at the bottom of thebottom component alignment mark. If the stacked components are to beplaced on a supporting plane, the alignment mark at the bottom of thebottom component can be connected to a ground reference point on thesupporting plane to create a monitoring path from probe to ground. Asanother option, a pair of electrical alignment marks can be incorporatedon the components to be stacked, which can be connected to a conductionpath in the supporting plane for the probe to monitor the stackingaccuracy from the top of both alignment marks on the stacked components.The little circle in the figure indicates contact points to otherstacked components or to the supporting plane.

FIG. 18 shows a different way of using an electrical alignment method toalign two objects together. In this example, an array of four sensors isincorporated in the first object and an array of four triangles as areference target is incorporated in the second object for monitoring thealignment accuracy. The four reference triangles can be separated asfour reference marks or can be abutted together to form a singlereference mark as shown. The four sensors can be a set of resistanceprobes for use in a contact measurement or can be a set of capacitanceprobes for use in a non-contact approach.

The setup works in a similar manner. Assume the first object is deviatedfrom the second object in the lower-left direction to cause the two leftalignment sensors on the first object to lie outside the quad-trianglereference mark associated with the second object. Assuming the range ofthe sensor array and the range of the reference array are matched, aninstruction to shift the first object to the right could be generated bythe sensing electronics. After the first object is shifted to the right,the two lower sensors would then step beyond the reference mark region(triangles C, D) on the second object, but the two upper sensors wouldstill be within the range of reference triangles at this moment. Bymoving the first object upward, i.e., toward the direction that thesensors have detected currents, the four alignment sensors can then bemoved into the range of the reference mark and the two objects arefinally aligned.

Alignment Using Optical Probe

If the alignment marking on the top of a component and the referencemarking on a target platform are highly reflective, an optical probe canbe used for the component alignment. The alignment mark can also be atransparent optical path 1901 through a component that enables a laserbeam to pass through and reflect back from a reference mark underneath,as shown in FIG. 19. Either a reflective surface marking on top of acomponent or an optical path through a component can be used as anoptical alignment mark.

In optical probing, there is no need to ground the alignment mark or theoptical path on a component, nor is there a need to ground the referencemark on a target platform. However, the spatial relationship between thealignment mark on the top and the contact array on the bottom of acomponent should match the spatial relationship between the referencemark and the associated land pattern on a target platform, as in otherprobing techniques.

FIG. 20 shows a set of photo-detectors or photo-diodes grouped in anarray to form a multiple-sensor optical probe for non-contact probing.The sensors in the array can be selectively grouped per the resolutionrequirements. For example, FIG. 20 shows a sixteen-sensor probe. But ifthe boundary of the reflection beam is out of focus and is slightlyblurred or if the size of the reflection beam is rather large, then thesixteen photo-detectors on the optical probe could be rearranged intofour groups to form a four-senor optical probe. The re-arrangement ofsensors can be done by re-arranging the input signals to the probeelectronics. In a different example, if the incident beam can beadjusted and focused, only the four sensors at the center of the probecould be chosen as the target sensors of alignment to increase thealignment accuracy, although the entire range of all 16 sensors are usedas active sensors to increase the incident beam search range.

If a generalized test probe, consisting of an array of test sensors, isa resistance probe, it measures the connection resistance or DC currentat each contact point. If it is a capacitance probe, it measures the ACcurrent at each test sensor, which reflects the area overlapping betweenthe surface of each sensor and the target reference. If it is an opticalprobe, it detects the photocurrent at each photo-detector, whichindicates the position of the reflection beam.

Using optical alignment, the probe position must be aligned with thecenter of a reference mark on a target platform at the beginning ofcomponent placement. The reference mark can be a simple reflective padin optical alignment. Ideally, the diameter of an incident light beamshall be compatible with the size of the reference mark. Otherwise, anuncertainty takes place, regardless of whether the size of the incidentlaser beam is too large or the size of the reference mark is too large.On the detector side, the incident beam shall be properly focused toensure it is within the range of the optical detectors. After aligningthe probe to the center of the reference mark, a component is moved inand placed at the land pattern. Next, the alignment mark is aligned withrespect to the position of the probe. For non-contact probing, thistwo-step indirect alignment method determines the ultimate accuracy incomponent placement. Depending upon how the light beam is collimated inthe optical approach, the relative sizes of the alignment mark and thereference mark are not as critical as in the case of resistance probing.

FIG. 21 shows an example of an optical pickup containing afour-photo-detector probe and associated current-voltage (I/V)amplifiers for detecting the deviation of displaced components. Each I/Vamplifier includes a current-voltage converter and a voltage amplifier.As light incidents on the photo-detector on a probe, a current isgenerated, which is then amplified by the I/V amplifier. A circularlight beam 2002, reflected from a target reference is also shown. Thereflected beam will be at the center of the optical pickup if thecomponent is fully aligned to the reference target.

Four different voltages, VA, VB, VC, and VD, are generated by theoptical pickup in FIG. 21. A set of comparators can be used to comparethe voltage output from these I/V amplifiers. The direction ofadjustment can be determined by comparing the output voltage of each I/Vamplifier with a predetermined threshold voltage. A high output voltagedetected by the comparator indicates the probe and the target referencehas area overlap. A low output voltage detected by the comparatorindicates the corresponding area is still misaligned. By shifting thecomponent from the position where the comparator detects a high outputvoltage to the position where the comparator detects a low outputvoltage, the misalignment can be fixed. If none of the comparatorsdetect a voltage level that exceeds a pre-determined threshold level(VT), i.e., no relevant reflection beam is detected by any of fourphoto-detectors, then the component is entirely misplaced or the opticalprobe is not in line with the target reference at all. The component isaccurately aligned on the target platform if all four voltage levelsreach the pre-determined threshold level.

Alignment Procedure

The alignment procedure to improve the accuracy in component placementis summarized as follows.

a. In preparation for component placement in assembly, some relevantinformation, such as the coordinates of the center of a land pattern orthe coordinates of a reference mark associated with the land pattern ona target platform, the probe configuration, the size of an alignmentmark on a component to be placed, and so on, are entered into thedatabase of a placement system. The information is useful in theplacement automation.

b. Load a target platform onto assembly equipment. A reference point onthe target platform is chosen as the origin to match the origin of thecoordinate system stored in the placement database.

c. The probe can be a single-sensor probe for monitoring the placementaccuracy, or it can be a multiple-sensor probe for fixing the deviationof a displaced component. To fix the deviation of a component notaccurately placed, the placement system needs to align the probe to thecenter of a target reference mark prior to placing a component on atarget platform. The probe can be a resistance probe, which measures theon/off conduction from probe sensor, through an alignment mark, to areference mark via a direct contact approach. The probe can be acapacitance probe, which detects the extent of area overlap between theprobe and the target reference via a non-contact approach. The probe canalso be an optical probe, which measures optical alignment between theprobe and a target reference.

Aligning a probe to the center of a reference mark can be done by movingthe probe to the coordinates of the reference mark per the informationin the placement system database and adjusting the probe position, ifnecessary, until all sensors detecting current flow are the innersensors around the center of the probe. For a resistance probe, theprobe must be lowered down to make a direct contact with the referencemark. For a no-direct-contact capacitance probe, the probe needs to moveto the range such that no foreign conduction object is within the rangeof its electrical field. For an optical probe, the probe needs to beheight adjusted to have the light beam match the diameter of the targetreference.

The inner sensors detecting current flow, based on the measurementthrough a reference mark, determine the range of active sensors on theprobe. With proper adjustment the center of these active sensors canmatch the center of the reference mark automatically. The center and therange of active sensors on the probe are the new position and target forthe alignment mark on the component to be aligned to. The multiple-senorprobe is able to provide feedback for component position adjustment,which is different from a single-sensor probe which can only detect theplacement accuracy.

d. Pick up a component and place it at a target land pattern. Theplacement is based on the coordinates of the center of the target landpattern or the coordinates of the reference mark.

e. Adjust the component pickup header until the position of thecomponent alignment mark is in line with the probe. The direction ofadjustment is in the direction to make more active sensors detectcurrent flows. The alignment is achieved when the alignment mark is inthe position that all effective active sensors on probe can detect itspresence.

Since the spatial relationship between the reference mark and the landpattern on the target platform can be pre-fabricated to match thespatial relationship between the component's alignment mark and itscontact array, the component can then be well aligned to the target landpattern. Some minor out-of-specification components could be also usedin assembly using such alignment technology.

f. For components with embedded indirect alignment paths, all indirectalignment paths can be connected in a daisy chain to form a serialalignment chain or several shorter alignment chains. The alignment chaincan be used to detect the connection status for all components in thechain when the system is put in use.

The above sequence of steps illustrates a method according to anembodiment of the present invention. Other alternatives can also beprovided where steps are added, one or more steps are removed, or one ormore steps are provided in a different sequence without departing fromthe scope of the claims herein.

The alignment technique is not limited to the placement of components onPCBs in assembly. It can also be used in a variety of applications, suchas the monitoring of the chip packaging, where the die to be mountedonto a substrate is viewed as the component and the substrate of thepackage is viewed as the target platform. Eventually, the alignment markand probing technique can be applied to multiple package stacking, baredie stacking, multi-chip carrier module assembly, module assembly,encapsulated card assembly, motherboard assembly, and so on. One ofordinary skills in the art would recognize many variations,modifications, and alternatives.

1. An assembled multi-component electronic apparatus comprising: a firstcomponent including: a first surface, a second surface, at least onealignment mark at the second surface of the first component, the atleast one alignment mark having a plurality of sensors, and a pluralityof interface contacts on the second surface of the first component, theat least one alignment mark having a spatial relationship to theplurality of interface contacts of the first component; and a secondcomponent including: a first surface, at least one reference mark at thefirst surface of the second component, and a plurality of interfacecontacts on the first surface of the second component, the at least onereference mark having a spatial relationship to the plurality ofinterface contacts of the second component, wherein when the at leastone reference mark is in a first position with respect to the pluralityof sensors, each of the plurality of sensors provides a conduction paththrough the at least one reference mark when terminated by an externalprobe, and wherein the plurality of interface contacts of the firstcomponent are aligned to the plurality of interface contacts of thesecond component when the at least one reference mark is in the firstposition with respect to the plurality of sensors.
 2. The assembledmulti-component electronic apparatus of claim 1, wherein the at leastone reference mark is larger than each of the plurality of sensors. 3.The assembled multi-component electronic apparatus of claim 1, whereinthe reference mark overlaps each of the plurality of sensors when the atleast one reference mark is in the first position with respect to theplurality of sensors.
 4. The assembled multi-component electronicapparatus of claim 1, wherein the conduction path is coupled to power orground.
 5. The assembled multi-component electronic apparatus of claim1, wherein the conduction path is terminated at the first surface of thefirst component.
 6. The assembled multi-component electronic apparatusof claim 1, wherein the conduction path is terminated at the firstsurface of the second component.
 7. The assembled multi-componentelectronic apparatus of claim 1, wherein the conduction path isterminated at the first surface of the first component and at the firstsurface of the second component.
 8. The assembled multi-componentelectronic apparatus of claim 7, wherein a portion of the conductionpath is within the first component, on the first surface of the firstcomponent, or on the second surface of the first component.
 9. Theassembled multi-component electronic apparatus of claim 7, wherein aportion of the conduction path is within the second component, on thefirst surface of the second component, or on the second surface of thesecond component.
 10. The assembled multi-component electronic apparatusof claim 1, wherein the second component includes a second surface. 11.The assembled multi-component electronic apparatus of claim 10, whereinthe conduction path is terminated at the first surface of the firstcomponent and at the second surface of the second component.
 12. Theassembled multi-component electronic apparatus of claim 10, wherein theconduction path is terminated at the first surface of the secondcomponent and at the second surface of the second component.
 13. Theassembled multi-component electronic apparatus of claim 10, wherein theconduction path is terminated at the second surface of the secondcomponent.
 14. The assembled multi-component electronic apparatus ofclaim 1, wherein the conduction path is adapted to couple a currentbetween the at least one reference mark and one of the plurality ofsensors.
 15. The assembled multi-component electronic apparatus of claim14, wherein the current is a direct current (DC) and one of theplurality of sensors being in contact with the at least one referencemark.
 16. The assembled multi-component electronic apparatus of claim14, wherein the current is an alternating current (AC) and one of theplurality of sensors being capacitively coupled to the at least onereference mark.
 17. The assembled multi-component electronic apparatusof claim 1, wherein the conduction path is adapted to couple an opticalsignal between the at least one reference mark and one of the pluralityof sensors.
 18. The assembled multi-component electronic apparatus ofclaim 1, wherein the first component is a printed circuit board, asystem mother board, a flex circuit, an optical device, an electronicsubstrate, a ceramic board, an integrated circuit bare die, a stackedintegrated circuit die, a stacked integrated circuit chip, an integratedcircuit encapsulated in a package, or an electronic device laminatedwith anisotropic conducting elastomer at a surface of one of theplurality of sensors.
 19. The assembled multi-component electronicapparatus of claim 1, wherein the second component is a printed circuitboard, a system mother board, a flex circuit, an optical device, anelectronic substrate, a ceramic board, an integrated circuit bare die, astacked integrated circuit die, a stacked integrated circuit chip, anintegrated circuit encapsulated in a package, or an electronic devicelaminated with anisotropic conducting elastomer at a surface of one ofthe plurality of sensors.